The invention concerns an arrangement for operating a continuously variable transmission for a motor vehicle, in particular a CVT automatic transmission, having a primary pulley pair axially adjustable relative to each other, a secondary pulley pair axially adjustable relative to each other and a flexible power-transmitting element wound around the two, comprising a hydraulic circuit supplied by a hydraulic pressure source having attached to a first branch of the hydraulic circuit, two parallel second branches, each supplying pressure to a hydraulic actuator of a separate one of the pulley pairs wherein with the first branch and with each second branch is associated one pressure valve system independently controllable by a control unit for adjusting the hydraulic pressure in the relevant branch.
Such an operating arrangement is known, e.g. from DE 195 33 976. In the known solution, a controllable pressure-reducing valve is connected between each of the second branches and the first branch of the hydraulic circuit. The pressure in the first branch is adjustable by means of a controllable pressure-limiting valve. The two pressure-reducing valves and the pressure-limiting valve are each controllable by means of an electromagnetically operable proportional pressure-regulating valve. The pressure requirement on the hydraulic actuators for both pulley pairs, which changes according to the operating mode of the vehicle, is always met by an adjustment adapted to the momentary requirement of the output-side pressure level of the pressure-reducing valves by means of the appertaining proportional pressure-regulating valves. The pressure in the first branch, that is, the system pressure, is maintained by control of the pressure-limiting valve always above the respective high pressure existing in both second branches by an adjustable pressure offset.
The problem to be solved by this invention is to make available an arrangement for operating a CVT transmission of a motor vehicle, in particular a CVT automatic transmission, which compared to the cited prior art is improved with regard to the dynamic behavior of the hydraulic system and to the tendency to leakage and vibrations of the whole hydraulic circuit.
For the purpose the invention provides that at least one of the pressure-reducing valve systems associated with the second branches comprises a controllable pressure-differential valve connected between the first branch and the second branch concerned, and that the control unit be designed so as optionally to adjust in a first pressure-control mode in which it adjusts the pressure in the second branch attached to the pressure-differential valve by control of the pressure valve system associated with the first branch without control of the pressure-differential valve, or to work in a second pressure control mode in which it adjusts the pressure by control of the pressure-differential valvexe2x80x94if desired under simultaneous control of the pressure valve arrangement associated with the first branch.
In the inventive solution, in the first pressure control mode, the pressure in the second branch attached to the pressure-differential valve is coupled with the pressure in the first branch. If the pressure in the first branch is changed, the pressure in the second branch concerned follows maintaining a steady pressure difference with the pressure in the first branch.
It has been shown that by the direct coupling of the pressure in the second branch concerned with the pressure in the first branch, a clear reduction of vibration of the whole hydraulic circuit can be achieved. Also quicker response times are possible in the operation of the hydraulic actuator supplied from the second branch concerned and therewith a better dynamic behavior.
In current CVT transmissions or CVT automatic transmissions, because of constructional and geometric conditions, the contact pressure of one of the pulley pairs must often be adjusted, over a very large range of driving modes of the vehicle, higher than the contact pressure of the other pulley pair. This situation can be used to allow the control unit work during comparatively long operating phases in the first pressure control mode with the result of a super-dynamic and very poor vibration operation of the transmission.
In many current continuously variable transmission, only during relatively short or/and rare operation phases must the contact pressure of the other pulley pair be raised above that of one pulley pair. For these operation phases, the control unit can change to the second pressure control mode in which it can adjust the pressure in the second branch attaching to the pressure-differential valve independently of the pressure adjustment of the first branch.
In the practice it is often the secondary pulley pair whose actuator during many operation phases must be supplied with higher pressure than the actuator of the primary pulley pair. Then at least the pressure valve system of the second branch associated with the secondary pulley pair conveniently comprises a controllable pressure-differential valve. In particular each pressure valve system associated with one of the second branches can comprise a controllable pressure-differential valve so that both the actuator of the primary pulley pair and the actuator of the secondary pulley airxe2x80x94at the same time or at different timesxe2x80x94be supplied with a pressure coupled with the pressure of the first branch.
According to a preferred development, the pressure-differential valve can be controlled by an electromagnetically actuatable proportional pressure valve wherein the control unit is designed so as to operate in the first pressure control mode the proportional pressure valve in an end position in which the pressure is minimal in one control line leading from the proportional pressure valve to the pressure-differential valve.
In this manner can be extensively reduced, if not eliminated, in the first pressure control mode, leakage losses of the proportional pressure valve which can be considerable in the pressure-regulating range of the valve.
So essential improvements in efficiency can be obtained specially when the first pressure control mode, compared to the second pressure mode, is the temporarily predominant manner of operation of the control unit.
Other advantages and advantageous developments of the invention result form the claims and the embodiment fundamentally described herebelow with reference to the drawing.